AN2009 datasheet(33/39 Pages) STMICROELECTRONICS

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PWM MANAGEMENT FOR 3-PHASE BLDC MOTOR DRIVES USING THE ST7MC

– The potential of M is close to 2HV/3

Now depending on the switch we are going to apply the PWM signal ON, the demagnetization

time of winding C will be decreased and the end of demagnetization event will be accelerated

during this PWM signal OFF time. The goal is to apply the maximum reverse voltage on

winding C.

Figure 23. Acceleration of demagnetization during high side switch PWM off time

In our example, during step

Σ3 (switches T2-T1 are ON), the VMC voltage on the MC winding

was negative. At the beginning of step

Σ4 (switches T2-T3 are ON), the voltage on terminal C

will be tied to HV and the shortest demagnetization configuration is the one that provides the

lowest voltage at point M to apply the maximum reverse voltage and decrease the demagnet-

ization time.

It is easy to verify that the lowest voltage HV/3 is obtained when the PWM is applied only on

the high side switch T2. During the OFF time of this PWM signal, the voltage at point M is HV/

3 (configuration shown in Figure 23). Otherwise if the PWM signal is applied on the low side

switch T3, then during the OFF time of the PWM, the voltage at point M is HV. So, in this con-

figuration, the PWM signal has to be applied on the high side switch T2 after the commutation

event in order to accelerate the demagnetization time.

PWM on T2

demagnetization

current

Normal mode

magnetization

current

Normal

current

AN2009 Datasheet(PDF) 33 Page - STMicroelectronics